Every moving mobile node that is connected to a network may perform a handover to a new network at the time when it leaves the coverage area of the old one to sustain connectivity. If the mobile node has an ongoing data session over the connection than the connection will brake at least for the time of the handover process. Additional mechanisms like MobileIP can allow rerouting the traffic to the new point of connection so the session may be resumed. However, the handover duration is the lower time limit for the session discontinuity.
It is therefore appreciated to keep the handover duration as short as possible. A mechanism to preserve this is pro-active context transfer. Pro-active Context-Transfer allows establishing a session state in an access router (AR) or access point (AP), before the mobile node (mobile terminal) starts a handover to new network. The transfer is done through the backbone network the access router or access point is connected to. This could be for example the Internet.
The origin of the transferred data is an entity that has already knowledge about the context. In an alternative solution for the pro-active context transfer, the so-called reactive context transfer, the transfer of the context is started when the handover already has begun.
An important function to realize a pro-active context transfer is the selection of candidates to which the context is transferred before performing the handover. It is normally not predictable, which access router or access point is the next point of connection as the movement pattern of the mobile node is unknown. Generally, it is assumed that the cell coverage areas of the access points overlap such that a handover may be performed. However, if there are multiple new access points in reach this also gives only little help for the selection process of the new network to join.
Seamoby CTP and CARD
The IETF (Internet Engineering Task Force) working group Seamoby has developed two protocols concerning context transfer. These are the context transfer Protocol (CTP, see Loughney et al., “Context Transfer Protocol”, Internet Draft, October 2003, all Internet Drafts and RFCs available at http://www.ietf.org) and the Candidate access router Discovery (CARD, see Liebsch et al., “Candidate Access Router Discovery”, Internet Draft, September 2003). CTP serves as the protocol to initiate the context transfer and to carry the context data. Three parties are involved in the CTP communication, the mobile node (mobile node), the previous access router (pAR) and the next access router (nAR). All three, with different message types, can initiate the protocol exchange. If mobile node wants to or has to change its point of network access it sends a request at least to the next access router, in case it is already disconnected from previous access router. If the mobile node uses some sort of next access router prediction, maybe through CARD, it even sends a message to the previous access router starting context transfer to the predicted next access router before connecting to next access router itself. It contains the IP address of the next access router, mobile node's old IP address on previous access router, a list of to be transferred context data, the possibly known IP address on next access router and a flag requesting secure and/or reliable transfer of context. The context data, called “feature context”, is then sent in a further message.
The CARD protocol consists of only two messages, the CARD request and the CARD reply. They even can be used between two access routers, the possibly next access router (nAR) and the current access router, called previous access router (pAR), or between a mobile node and a previous access router or next access router. Between access routers, CARD helps to get capability information of the next access router candidates that is needed to select the most suitable one for context transfer and later mobile node handover. Between mobile node and previous access router a CARD request is issued to demand a list of next access router candidates. In this request the mobile node can send any next access router data link layer (Layer 2) identifiers it might have detected by some mechanism, so the previous access router has a hint, which access router is in range of the mobile node. The way the previous access router identifies a next access router by its Layer 2 identifier is not specified in the CARD draft. In the reply to the mobile node previous access router sends a next access router list with the belonging next access router capabilities that could have been pre-filtered by a previous access router determined criteria to reduce the number of next access routers the mobile node has to process as candidates.
Context Transfer in Wireless Local Area Networks (IEEE 802.11f)
In Wireless Local Area networks (WLAN) information about the client or station (STA) between the access points (AP) involved in the handover allowing the re-association process at the new access point are exchanged. A context transfer scheme to accelerate this re-association process is used. Two functional entities, the access point and the RADIUS server are involved in the context transfer. For the station (STA) the management process is transparent. The RADIUS server fulfils the task of mapping delivered Basic Service Set Identifiers (BSSID) to IP addresses or Fully Qualified Domain Names (FQDN) of access points. This mapping implicitly shows if an access point belongs to the same extended service set (ESS) as the RADIUS server. It also distributes on request cipher keys to the access points to allow encrypted communication between two access points. The communication includes all management data that allows the movement of clients between the nodes and enforces the association of a client only with one access point at a time. The management messages can contain context data. Each access point in an ESS following maintains a dynamic representation of its neighboring access points. This representation is also referred to as the Neighbor Graph.
Important to note is that in the IEEE Draft IEEE 802.11f-D3 “Recommended Practice for Multi-Vendor Access Point; Interoperability via an Inter-Access Point Protocol Across Distribution Systems Supporting IEEE 802.11 Operation”, January 2002, states in the annex B, section B.3.1, that context transfers between media with different service models should not be expected to be successful. Attempts to transfer context between cellular devices and IEEE 802.11 access points according to the IEEE 802.11 context transfer mechanism will fail unless the cellular access points implement the same set of services as the 802.11 access points. In conclusion the document states that context transfers between heterogeneous technologies will fail.
Other Mechanisms
Additionally to the access router and mobile node of the CTP scenario, the mechanism described in US 2003/0,103,496 A1 comprises a Policy Server (PS) that serves the task of retrieving the neighboring access networks (AN) and access routers capable of a context transfer (comparable to CARD mechanism). The access networks are indicated by Layer 2 information in beacon signals, received by the mobile node, and looked up in a local database by the PS. The PS communicates with neighboring Policy Servers if they are capable to serve the mobile node and pre-authenticates it with them. One drawback of this mechanism is that it needs even more secured connections (or Security Associations in other terms) than the CTP scenario. Next, it does construct the context from a dynamic and static part before sending to the new access network, but it does not take into account any features or capabilities of the target network.
US 2003/0,092,444 A1 describes a mechanism that takes dynamic parameters like current traffic load and user rights into account for selection of neighboring candidates. The list of candidates may differ therefore for each mobile node. The transfer process itself is performed between the access routers of the access networks.
A mechanism for discovery of neighboring access routers, useable for a context transfer mechanism, is presented in US 2003/0,087,646 A1 and is abbreviated GAARD. It allows detecting geographically neighboring access networks even if they are not topologically neighbors considering e.g. IP addresses. In this document, a mobile node has a local cache of Layer 2 (data link layer) addresses and Layer 3 (network layer) addresses. When the mobile node receives a beacon signal with a Layer 2 address and wants to initiate a context transfer to this node, it looks up the corresponding Layer 3 address in its cache. If the cache lookup fails, the mobile node requests the serving access router to lookup the corresponding Layer 3 address. The access router itself looks up its cache. If the address is not found, it starts a dynamic discovery process to derive the requested Layer 3 address. The access router returns the address to the mobile node that in turn uses it to start a context transfer or other handover mechanisms to the identified access router. The functionalities of this system must exist in every access router and mobile node that wants to use or support this system. The mechanism can serve as an implementation of the CARD process.
In contrast to the generic context transfer scenario, where all neighboring access routers are even possible points of access for a mobile node, the latter assumption is not true in a scenario of networks, working together on a contract base. A neighboring access router may belong to a network operator without a roaming agreement with the mobile node's home operator. So the mobile node receives beacons of the foreign network but an authentication process will fail, as the Authentication Authority in the foreign network is unable to connect to the home AAA (authentification, authorization and accounting) server of the mobile node or at least will not trust this unknown server. A context transfer to such a network will also fail for the same reason.
A way to integrate networks without a direct roaming agreement with the mobile node's home operator is the use of proxy AAA servers. The access network operator trusts the proxy AAA server of an operator that itself trusts the home operator of the mobile node. This way the mobile node can be authorized even in this foreign network.
It is probable that mechanisms like the context transfer in WLAN are developed also for other local area network technologies. Context transfer between topologically adjacent entities has the advantage of short distance, and this way low latency transfers.
Associated with the previous issue is the fact of a heterogeneous access network structure. It is very likely that a moving mobile node's context cannot be regarded as static data. It will change as the access network infrastructure of the new point of access differs from the previous one. The simple forwarding of context data to the new access network will not solve this issue.
A general problem of context transfer is the trust relationship between the entities involved in the context transfer process. In a scenario with an area containing a number of n neighboring access routers, transferring context data between each other, this gives an upper bound of
      n    ⁡          (              n        -        1            )        2trust relationships between all the access routers. As these relationships are technically represented by some cipher key exchange between peers to allow encrypted communication, a large number n means a lot of storage space for key data sets, in the example n−1 data sets per access router. Also these relationships must be established before context transfer is possible between the peers, requiring a management function. A method to reduce the number of trust relationships would therefore save storage space and management effort. An already existing technology for secure data transport is IPSec naming the trust relationships Security Associations (SA).
A mobile node, in most cases, will be connected to its point of access by a wireless link. These links normally have lower bandwidth than wired links in the backbone part of the access network and the interconnection of the different access networks. This leads to higher transfer cost per amount of data in the wireless domain.
Another aspect is the power consumption of the mobile node. The consumption is directly correlated with the number of send packets to the wireless link. Both aspects lead to the objective to keep the amount of management traffic as low as possible, compared to the user payload traffic.